Send & Receive Issues

How to Resolve SMTP Error 550 5.7.1 (Client Host Blocked by Spamhaus)

Huda Qurayshi Oct 1, 2026 868 views

Question details

Users are unable to send emails and receive a bounce-back message indicating SMTP Error 550 5.7.1, caused by the sending server's IP address being blocked by Spamhaus.

How to Resolve SMTP Error 550 5.7.1 (Client Host Blocked by Spamhaus)

Product: Microsoft 365 / Email Client
Device & OS: not provided
Scenario: Sending outbound emails to external recipients.
Observed behavior: The receiving server rejects the email, returning a 550 5.7.1 NDR error because the sending host or IP is listed on a Spamhaus blocklist.

Before you start

Before proceeding, determine your public-facing sending IP address and check if emails are failing for all external domains or just one specific recipient.

Solution 1Recommended

Check Spamhaus Blocklist and Request Delisting

The most direct way to resolve this NDR error is to verify if your IP is actually on the Spamhaus blocklist and request its removal.

Spamhaus maintains several databases of IP addresses associated with spam. If your sending IP is on one of these lists, most major email providers will automatically reject your messages.

Identify your sending IP address

Find the public IP address of your email server or your Microsoft 365 tenant's outbound IP from the NDR message headers.

Check the Spamhaus Blocklist

Visit the official Spamhaus Project website and navigate to the IP and Domain Reputation Checker tool. Enter your sending IP address to search their database.

Follow delisting instructions

If your IP is listed, click on the specific blocklist entry and carefully follow the provided instructions to request removal. You may need to secure your mail server before the request is approved.

Check Spamhaus Blocklist and Request Delisting

Delisting Timeframe: Once a delisting request is submitted and accepted, it may take up to 24 hours for the changes to propagate across all recipient email servers globally.

Solution 2

Verify Outbound SMTP Settings and Recipient Platform

Check your email client's configuration and determine if the issue is isolated to a single recipient's strict server policies.

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Frequently Asked Questions

What does SMTP Error 550 5.7.1 mean?

This error means that the recipient's mail server refused to accept your email. The '5.7.1' specifically points to a security or policy violation, which in this case indicates your sending IP or domain was blocked by a spam filter like Spamhaus.

Why is my email blocked even if the recipient allow-listed my address?

Spamhaus and similar blocklists operate at the server level (Mail Transfer Agent). The receiving server will drop the connection based on the IP address before it even looks at the recipient's personal inbox allow-list.

How do I resolve NDR errors in Microsoft 365?

If you are using Microsoft 365, you should check the Microsoft Defender portal. Navigate to the 'Restricted entities' page to see if your user account or tenant has been blocked from sending outbound mail, and follow the prompts to unblock it.

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